1. Field of the Invention
The present invention relates to a resonant-type switching power supply device. Specifically, the present invention relates to prevention of a through-current in a switching power supply device.
2. Description of the Related Art
FIG. 1 is a circuit configuration diagram of a conventional resonant-type switching power supply device. In FIG. 1, a full-wave rectifier circuit 2 (which corresponds to an input rectifier circuit) rectifies an alternating current of an alternating current power supply 1 for commercial use to output a full-wave rectifying voltage to a smoothing capacitor 3. The smoothing capacitor 3 obtains a direct current power supply Vin by smoothing the full-wave rectifying voltage of the full-wave rectifier circuit 2.
At both ends of the smoothing capacitor 3, a series circuit including a switching element Q1 consisting of a MOSFET or the like and a switching element Q2 consisting of a MOSFET or the like is connected.
The switching element Q2 is connected in parallel to a series resonant circuit consisting of a reactor Lr, a primary winding P1 (winding number N1) of a transformer T1 and a current resonant capacitor Cri, and a voltage resonant capacitor Crv. The reactor Lr may be a leakage inductance between the primary winding P1 and a secondary winding S of the transformer T1.
The primary winding P1 and a secondary winding S (winding number N2) of the transformer T1 are wound so as to generate a reverse phase voltage with respect to one another. A rectifying and smoothing circuit consisting of a rectifier D0 and a smoothing capacitor C0 is connected to the secondary winding S of the transformer T1. This rectifying and smoothing circuit rectifies and smoothes a voltage (pulse voltage which is controlled to be on/off) induced across the secondary winding S of the transformer T1 to output a direct current output Vo to a load not shown.
A feedback circuit 5 is connected to a connecting point of the smoothing capacitor C0 and the rectifier D0, and detects an output voltage of the smoothing capacitor C0 to output a detecting signal to a control circuit 7. The control circuit 7 controls the voltage of the load to be constant by alternately turning on/off the switching element Q1 and the switching element Q2 by pulse width modulation (PWM) control based on the detected voltage from the feedback circuit 5. In this case, voltage having a dead time is applied to each gate of the switching element Q1 and the switching element Q2 so as to alternately turn on/off the switching element Q1 and the switching element Q2.
Next, the operation of the conventional resonant-type switching power supply device configured as described above will be described by referring to a timing chart of FIG. 2. FIG. 2 is a timing chart of a signal in each part when the conventional switching power supply device is in a stationary state.
It should be noted that in FIGS. 2 to 4, VQ1gs is a gate signal between a gate and source of the switching element Q1, and VQ2gs is a gate signal between a gate and source of the switching element Q2. VQ2ds is a voltage between the drain and source of the switching element Q2. IQ2 is a current flowing through the drain of the switching element Q2. IQ1 is a current flowing through the drain of the switching element Q1. ILri is a current flowing through the reactor Lr. Vcri is a voltage at both ends of the current resonant capacitor Cri. ID0 is a current flowing through the rectifier D0. In addition, with the dead time of around several 100 nS, the switching elements Q1 and Q2 are alternately turned on/off by the gate signals VQ1gs and VQ2gs.
First, in an on-period of the switching element Q1 (for example, times t11 and t12), energy is stored in the current resonant capacitor Cri through an exciting inductance of the primary winding P1 of the transformer T1 and the reactor Lr (the leakage inductance between the primary winding P1 and secondary winding S of the transformer T1).
Next, in an on-period of the switching element Q2 (for example, times t12 to t14), the energy stored in the current resonant capacitor Cri is transmitted to the secondary side of the transformer T1, and the exciting energy of the exciting inductance of the primary winding P1 is reset.
In the on-period of the switching element Q2, a voltage of the current resonant capacitor Cri that has been divided by the exciting inductance of the primary winding P1 and the reactor Lr is applied to the primary winding P1. When Vf is a forward voltage drop of the rectifier D0 and the voltage of the primary winding P1 becomes (Vo+Vf)×N1/N2, the voltage is clamped. Then, a resonant current by the current resonant capacitor Cri and the reactor Lr is transmitted to the secondary side of the transformer T1 so that a current ID0 flows through the rectifier D0. When the voltage of the primary winding P1 is less than (Vo+Vf)×N1/N2, the energy is not transmitted to the secondary side of the transformer T1 and the resonant operation is carried out only on the primary side of the transformer T1.
In this switching power supply device, the control circuit 7 controls an energy amount to be transmitted to the secondary side of the transformer T1 by changing the on-period of the switching element Q1 to change the voltage of the current resonant capacitor Cri. The on-period of the switching element Q2 is generally set by a time determined by the PWM control of the switching element Q1 or a resonant period for transmitting a current to the secondary side of the transformer T1 when frequencies are fixedly controlled.
In addition, just after the switching element Q1 is turned off (for example, just after time t12), an exciting current by the exciting inductance of the primary winding P1 and the reactor Lr flows through a body diode of the switching element Q2. Since the switching element Q2 is turned on during this period, zero voltage switching and zero current switching of the switching element Q2 can be carried out. Therefore, a switching loss is not caused.
When the switching element Q2 is off (for example, time t14), it is a period in which energy transmission to the secondary side of the transformer T1 is completed and only a cyclic current flows on the primary side of the transformer T1. Therefore, a peak of the current is low, and the switching loss is extremely small since a voltage quasi-resonant operation is carried out by the voltage resonant capacitor Crv. Just after the switching element Q2 is turned off, the cyclic current is regenerated to a direct current power supply Vin through the body diode of the switching element Q1. Since the switching element Q1 is turned on during this period, zero voltage switching and zero current switching of the switching element Q1 can be carried out. Therefore, a switching loss is not caused.
Meanwhile, in the switching power supply device, an output voltage is still low at the time of starting-up. In addition, when the output current becomes overloaded, the output voltage is generally lowered because electric power is limited due to over-current protection.
When the switching element Q2 is turned on, the cyclic current is generally set to be positive (the broken line portion of ILri in FIG. 2, and times t12 and t13) when the period for transmitting energy to the secondary side of the transformer T1 is completed. However, when the output voltage decreases at the time of starting-up or overloading, the voltage, which is applied to the primary winding P1 during the on-period of the switching element Q2, is clamped at a voltage lower than a general voltage. Therefore, a time required for resetting the exciting energy becomes longer, and the cyclic current is kept negative when energy transmission to the secondary side of the transformer T1 is completed (the broken line portion of ILri in FIG. 3, and times t22 and t23).
In addition, even in a power supply in which an over-current protection circuit is not provided and an output voltage does not decrease at the time of overloading, when frequencies are constant or the on-period of the switching element Q2 is determined by the energy transmission period to the secondary side of the transformer T1, the cyclic current is negatively superimposed in order to store larger energy to the current resonant capacitor Cri, and thus the cyclic current is kept negative when the energy transmission to the secondary side of the transformer T1 is completed.
In this state, the exciting energy of the primary winding P1 of the transformer T1 is not reset. In this time, the cyclic current flows in the reverse direction through the body diode of the switching element Q2, which is called resonance deviation. When the switching element Q1 is turned on in this state, the voltage Vin of the direct current power supply is applied to the body diode in the reverse direction and a reverse recovery current flows. In general, a body diode which is parasitically formed in a switching element takes a long time for reverse recovery, and thus large current flows therein. In the worst case, the circuit may be damaged.
In order to avoid this problem, it is only necessary to apply a voltage in the reverse direction when a current does not flow through the body diode. A method in which the switching element Q2 is turned off and the switching element Q1 is turned on while energy is transmitted to the secondary side of the transformer T1 (when the current of the switching element Q2 is positive), is possible. A timing chart of signals by this method is shown in FIG. 4.
However; when the switching element Q2 is turned off while the energy is transmitted to the secondary side of the transformer T1 (for example, time t33), just after that, the resonant operation of the reactor Lr and the voltage resonant capacitor Crv is caused with high frequencies by the exciting energy of the reactor Lr, and the current ILri makes sharp decline to a level of the cyclic current (for example, times t33 and t34).
Since the cyclic current in this time is negative (for example, time t34), a current also flows through the body diode. Therefore, when the switching element Q1 is turned on, a large reverse current flows. It is only necessary for the switching element Q1 to be turned on when the current is positive during the switching element Q2 is turned off and the current is decreasing to the cyclic current. However, in order to prevent the both switching elements from being turned on, a dead time is provided since the switching element Q2 is turned off until the switching element Q1 is turned on. Therefore, it is difficult that the switching element Q1 is turned on after the switching element Q2 is turned off during this rapid current change.
In addition, when the switching element Q2 is turned off during the energy is transmitted to the secondary side of the transformer T1, a loss is caused by the recovery current of the rectifier D0. In addition, since a surge current is generated, a snubber circuit has to be added by using a high voltage rectifier.
Moreover, a method for solving these problems is disclosed in Japanese Patent Laid-open Application no. 2005-51918. The switching power supply device disclosed in the Japanese Patent Laid-open Application no. 2005-51918 is configured that a current of a body diode is detected by a current state detecting circuit so that the switching elements Q1 and Q2 are not turned on/off while the current flows through the body diode.
However, in the switching power supply device disclosed in the Japanese Patent Laid-open Application no. 2005-51918, a loss is caused in the current state detecting circuit and efficiency is deteriorated.